Seed descriptions are not persisted on BigQuery

[drewbanin]

Describe the bug

When persist_docs is configured to persist descriptions for BigQuery relations, the resulting seed tables in BigQuery are not build with relation-level descriptions.

Steps To Reproduce

# dbt_project.yml
config-version: 2
seeds:
    +persist_docs:
      relation: true
      columns: true

# models/schema.yml
version: 2
seeds:
  - name: my_seed
    description: my seed
    columns:
      - name: the_column
        description: column description

$ dbt seed

The resulting relation is not configured with a description in BigQuery.

Expected behavior

I expect to see a configured description in BigQuery for the seed table.

System information

Which database are you using dbt with?

• postgres
• redshift
• bigquery
• snowflake
• other (specify: ____________)

The output of dbt --version:

dbt v0.17.1-rc2

[jml]

Note: the column descriptions are being persisted.

[beckjake]

We create seeds on bigquery via load_table_from_file, which doesn't let us set a description as far as I can tell. We'll have to add a second step that adds a comment after the fact.

[drewbanin]

fixed by #2601